Motor Stator and Motor

ABSTRACT

A motor stator in one implementation includes a plurality of teeth disposed in a radial form centered about a predetermined center axis, insulators covering the plurality of teeth, and a plurality of coils provided on each of said plurality of teeth by winding conductors onto said insulators through to multiple layers. At one axial end of the plurality of areas where said insulators cover each of said plurality of teeth, the radius of curvature of the conductor windings on the ending side is larger than the radius of curvature on the starting side. The insulators in a motor stator in another implementation are provided with a first partition and a second partition that, relative to the axial center of the plurality of teeth, respectively cover one side and the other side each tooth.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to electrically powered motors, and inparticular, to stators.

2. Description of the Related Art

Electrically powered motors to date have been furnished with a stator,and with magnetic-field producing magnets, which are the drive part thatrotates a motor's rotor section relative to its stator section. Statorsinclude an annular core, insulators as nonconducting components thatcover teeth sections of the core, and coils formed by wrappingconductors onto the insulators. In midsized motors such as are used for“electric power steering systems” (as termed in, for example, U.S.patent application Pub. No. 2002/0175574, but also referred to as“steer-by-wire systems”), conductors of relatively large diameter arewrapped around insulators and employed as the coils.

In such motors, owing to the fact that motor efficiency increases withconductor diameter, technology for improving the slot-fill factor of thecoils by wrapping on the conductors in regular rows without gaps isparamount. In the present specification, the term “space factor” (astermed in, for example, paragraph [0006] of both U.S. patent applicationPub. No. 2004/0263015 and patent application Pub. No. 2005/0029894, butalso referred to as “slot-fill factor,” or “slot-fill ratio”) isintended to mean the ratio of the area occupied by the conductors withrespect to the cross-sectional area in the interval (slot) betweenadjoining stator teeth.

In stator implementations employing large-diameter conductors that donot bend readily, when the conductors are wound onto the teeth, it canhappen that a conductor, as wrapped onto a tooth from one flank to theother flank, does not bend fully around a terminus of the tooth, and isinstead left riding up off the tooth, not lying in full contact alongthe flanks. For example, in stator implementations in which theinsulators are formed with tooth-flank guide grooves, it can happen thatthe conductors fail to be guided by the guide grooves, which isprohibitive not only of attaining evenly-rowed wrapping but also ofsmooth conductor windings, and limits improvement in the slot-fillfactor of the coils.

BRIEF SUMMARY OF THE INVENTION

The present invention makes it possible to build coils smoothly onto anelectric motor stator. The present invention also makes it possible toreduce the thickness of coils on stator insulator surfaces, and toprevent the conductors from separating from the insulators.

In one example of the present invention, an electric motor statorcomprises multiple teeth radially arrayed and centered on a specifiedcenter axis, insulators respectively covering said multiple teeth, andmultiple coils constructed by winding multiple layers of conductors onsaid multiple teeth starting from said insulators. In an top end sectionwhich covers an top end and forms one end in a direction along therespective center axes of said multiple teeth, said insulator isprovided with multiple upper guide grooves which are arrayed in parallelto one another at a pitch approximately equal to the diameter of saidconductor, and which restrict the winding position of said conductor.

In another example of the present invention, the motor stator isprovided with multiple teeth radially arrayed and centered on aspecified center axis, insulators respectively covering said multipleteeth, and multiple coils constructed by winding multiple layers ofconductors starting from said insulators. At the top end section whichcovers the top end which forms one end in a direction along therespective center axes of said multiple teeth, said insulators areprovided with a protruding wall which extends in the conductor laminardirection on said center axis side of said teeth, or on the oppositeside of said center axis, and which is convex toward said teeth. Saidprotruding wall is constructed along the winding direction of said topend portion of the coil first layer conductor, and contacts said firstlayer to restrict the winding position of said conductor.

In yet another example of the present invention, the motor statorcomprises multiple teeth radially arrayed and centered on a specifiedcenter axis; multiple teeth inside end sections disposed at therespective end sections on said center axis side of said multiple teeth,while opening on both sides of the teeth in a circumferential directionwith respect to the center of said center axis; insulators respectivelycovering said multiple teeth; and multiple coils constructed by windingmultiple layers of conductors starting at said insulators. Saidinsulators comprise inside end guide grooves which restrict the windingposition of said conductors at the inside end sections which cover thesurfaces on the side opposite to that of the respective said center axesof said multiple teeth inside end sections.

The present invention permits the smooth construction of coils. Inparticular, it enables the smooth construction of coils by preventingconductor mis-winding at the top end of the multiple teeth.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a vertical section illustrating a motor according to a firstembodiment of the present invention;

FIG. 2 is a plan view illustrating a stator;

FIG. 3 illustrates the stator in a bottom view;

FIG. 4 is a plan view depicting an insulator;

FIG. 5 depicts the insulator in an underside view;

FIG. 6 illustrates the insulator in a lateral view;

FIG. 7 is an oblique view of the insulator;

FIG. 8 is a partially sectional view depicting a first partition;

FIG. 9 is a partially sectional view illustrating a second partition;

FIG. 10 is a plan view showing a coil first layer;

FIG. 11 is a bottom view showing the coil first layer;

FIG. 12 is a plan view showing a coil second layer;

FIG. 13 is a diagram illustrating a cross-section through coils on theflanks of insulators, wherein a winding machine nozzle is represented inphantom;

FIG. 14 is a plan view depicting a motor insulator involving a secondembodiment of the present invention;

FIG. 15 illustrates the insulator in a bottom view;

FIG. 16 is a partially sectional view showing a first partition for amotor involving a third embodiment of the present invention;

FIG. 17 is a partially sectional view depicting a second partition; and

FIG. 18 is a fragmentary enlarged sectional view illustrating an endportion of the first partition.

DETAILED DESCRIPTION OF THE INVENTION

Below preferred embodiments of the present invention will be explainedwith reference to figures. In the descriptions that follow, referencesto the positional relationships of each of the parts, or to upper,lower, right and left directions, etc. will in all cases be for thepurpose of indicating positional relationships or direction in thediagrams, and will not indicate positional relationships or directionswith respect to the actual machine when assembled.

First Embodiment

Overall Motor Structure

FIG. 1 is a vertical section showing an electric inner rotor-type motor1 according to a first embodiment of the present invention. In thediagram, the motor 1 is covered by a cylindrical housing 11 with a smallopening on the (+Z) side and a large opening on the (−Z) side in thediagram, and by a cover plate 12, which covers all but the centersection of the opening on the (−Z) side. Ball bearings 131, 132 are fitrespectively onto the housing 11 (+Z) side opening and onto the coverplate 12 opening, and a shaft 21 is rotatably supported by the ballbearings 131, 132. In the explanation that follows, the (+Z) and (−Z)sides along the motor 1 center axis J1 are respectively described as theupper and lower sides for the sake of convenience, but there is norequirement that the center axis J1 necessarily conform to the directionof gravity.

A cylindrical rotor yoke 22 is attached to the shaft 21 inside thehousing 11, and a multipole magnetized field magnet 23 is affixed to theouter perimeter surface of the rotor yoke 22. The stator 3 is disposedso that the center axis J1 of the annular stator 3 coincides with thecenter axis of the shaft 21.

The stator 3 comprises a plurality of teeth 31, arranged in a radialfashion with center axis J1 as the center (that is, extending from theinner circumferential surface of the housing 11 toward the shaft 21 (andthe field magnet 23)); a plurality of insulators 32 respectivelycovering the plurality of teeth 31; and a plurality of coils 35,constructed by winding a conductor in multiple layers along insulators32 on each of the plurality of teeth 31. In other words, the pluralityof insulators 32 are provided respectively between the plurality ofteeth 31 and the plurality of coils 35. Each of the teeth 31 is elongatein the axial direction (that is, the Z direction), and the coils 35 areformed by winding a conductor in the axially along the outer peripheryof the insulator 32. Additional details of the stator 3 will bediscussed later.

On the cover plate 12 side of the stator 3, a base plate attachment part51 is affixed to the inner circumferential surface of the housing 11,and a circuit board 52 is attached to the base plate attachment part 51so as to be positioned in between the cover plate 12, and the fieldmagnet 23 and stator 3. The stator 3 and the circuit board 52 areelectrically connected through a cross-over wire attached to the baseplate attachment part 51.

In the motor, the shaft 21 and ball bearings 131, 132 perform the roleof a bearing mechanism to rotatably support the rotor yoke 22 relativeto the housing 11, centered on the center axis J1. The drive currentsupplied to the stator 3 through the circuit board 52 is controlled sothat rotational power is generated between the stator 3 and the fieldmagnet 23 centered on the center axis J1, and the shaft 21 rotatestogether with the rotor yoke 22.

A drive circuit for controlling the drive current supplied to the stator3 and a Hall sensor 521 is attached to the circuit board 52 at the axialbottom side of the field magnet 23. The direction of rotation androtational position of the field magnet 23 are detected by the Hallsensor 521 to control drive current to the stator 3. In other words, themotor 1 is what is known as a brushless motor.

Stator Structure

FIGS. 2 and 3 are a plan view and a bottom view showing the stator 3prior to being furnished with the coils 35 (see FIG. 1). In the stator3, the plurality of teeth 31 (9 teeth in the present embodiment)protrude radially inward from a cylindrical outside section 30, centeredon the center axis J1, and at the leading end portions of the teeth 31radially inward along each, teeth inner edge portions 311 are providedon either side of the teeth 31, circumferentially following the outsidesection 30 (that is, that the width in the width orientationperpendicular to the radial orientation centered on the center axis J1is wider than the width of the teeth 31). The length paralleling thecenter axis J1 (along the axial orientation) of each of the teeth 31 isgreater than the teeth 31 width, and both flanks of each of the teeth 31in the circumferential orientation centered on the center axis J1 areparallel to the center axis J1.

As shown in FIGS. 2 and 3, each of the teeth 31 is covered by aninsulator 32. Part of the rim surfaces of the outside section 30 on the(+Z) side and (−Z) side, and virtually the entire end surfaces on the(+Z) and (−Z) side of each tooth inner edge portion 311 are covered bythe insulators 32. The teeth 31, the outside section 30, and the teethinner edge portions 311 are formed by a plurality of silicon steelplates laminated in the Z orientation and shaped to correspond to theform of these components. The insulators 32 are provided in order toelectrically isolate the coils 35 from each tooth 31 and itssurroundings.

Each of the insulators 32, as shown in FIG. 2, is furnished with aplurality of first guide grooves 321 in the insulator upper end portionthat covers the upper end of the tooth 31 (that is, the end portion onthe (+Z) side, which forms one end in the axial direction). Arrayed inparallel at a pitch approximately equal to the diameter of the conductorthat forms the coils 35 (the diameter including the coating on theconductor is preferably greater than 1.0 mm and less than 2.0 mm—in thepresent embodiment it is 1.5 mm), these upper guide grooves govern thewind-on position of the conductor. In an insulator 32 focused on singly,the plurality of guide grooves 321 is provided angled with respect to animaginary plane (indicated by the dot-and-dash line in FIG. 2) 300parallel to the center axis J1 and perpendicular to the teeth 31. Thefirst guide grooves 321 singly are formed in an inverse “U” thatstretches over the end surface and both flanks of the tooth 31 on itsupper side. Here, the pitch of the first guide groove 321 refers topitch along the perpendicular to the first guide grooves 321 (indicatedby arrow A1 in FIG. 2).

Each of the insulators 32 is in turn, as shown in FIG. 3, furnished withsecond guide grooves 322 on its axial lower end portion (i.e., the endportion on the (−Z) side, opposite the area where the first guidegrooves 321 (cf. FIG. 2) are formed). Arrayed in parallel with respectto the above-described imaginary plane 300, the plurality of lower guidegrooves is formed to govern the wind-on position of the conductor. Thesecond guide grooves 322 singly are formed in an approximate U-shapethat stretches over the end surface and both flanks of the tooth 31 onits lower side. The pitch of the second guide groove 322—that is, thepitch along the perpendicular to the second guide grooves 322 (normal tothe imaginary plane 300)—is equal to the pitch of the first guide groove321.

FIGS. 4 through 7 are enlarged views showing the tooth 31 and insulator32 and vicinity located furthest to the (+Y) side in FIG. 2. FIG. 4 is aplan view of the insulator 32; FIG. 5 is a bottom view thereof; FIG. 6is a lateral view seen from the (+X) side heading in the (−X) direction;and FIG. 7 is an oblique view thereof.

When forming a coil 35 (cf. FIG. 1), a conductor is wound onto theinsulator 32 along the first guide grooves 321 from the (−X) side to the(+X) side at the upper end portion of the insulator 32 shown in FIG. 4.The first guide grooves 321 (−X) and (+X) sides will hereafter bereferred to respectively as “start point 3211” and “end point 3212.” Atthe lower end portion of the insulator 32 shown in FIG. 5, the conductoris wound onto the insulator 32 from the (+X) side to the (−X) side alongthe second guide grooves 322. The second guide groove 322 (−X) and (+X)sides will hereafter be referred to respectively as “start point 3221”and “end point 3222.”

As shown in FIG. 4, the difference in radial distance from the centeraxis J1 between the respective start points 3211 and end points 3212 onthe plurality of first guide grooves 321 is the same as the pitch of thefirst guide grooves 321 and the second guide grooves 322 (in otherwords, it is approximately the same as the diameter of the conductor).It will be appreciated that this just-described difference in radialdistances is, precisely, the difference in vertical plumb distances froma plane that includes the center axis J1 in FIG. 2 and that is parallelto the imaginary plane 300; it will be referred below to as the firstguide groove 321 “inclination.” When the inclination is such that thestart points 3211 are further away from the center axis J1 than the endpoints 3212, it is deemed to be positive. The angle of inclination ofthe plurality of first guide grooves 321 with respect to theirrespective imaginary planes 300 (FIG. 2) is preferably greater than 5°and less than 20° (10.9° in the present embodiment). As shown in FIG. 5,the distances of the respective start points 3221 and end points 3222 onthe plurality of second guide grooves 322 from the center axis J1 areequal (that is, the inclination of the second guide groove 322 is zero).

As shown in FIGS. 4 and 5, for each of the respective plurality of firstguide grooves 321 and second guide grooves 322 in the insulator 32, theend point 3212 of a given first guide groove 321, and the start point3221 of a given second guide groove 322 are positioned along an axiallyextending straight line (that is, along a straight line parallel to thecenter axis J1, which extends in the Z orientation); and the end point3222 of the given second guide groove 322 and the start point 3211 ofthat other first guide groove 321 that is adjacent to the given firstguide groove 321 on its (−Y) side are positioned along an axiallyextending straight line.

In each of the insulators 32, in the inner edge portions that cover thesurface on the radially outward side of the tooth inner edge portions311, inner-edge-portion guide grooves 325 are furnished on both sides ofthe tooth 31, and on the upper- and lower-end tooth inner edge portions311, to govern the conductor wind-on position of the coils 35.

As shown in FIGS. 6 and 7, the insulators 32 are provided with a firstpartition 323, which is a molded resin part in which plural first guidegrooves 321 are formed, and a second partition 324, which is a moldedresin part in which plural second guide grooves 322 are formed; thefirst partition 323 and the second partition 324 are installed throughthe (+Z) and (−Z) sides of the teeth 31 (cf. FIG. 2).

The first partitions 323 cover the upper-end surface of the teeth 31,both flanks of the parts of the teeth 31 above the center in the axialorientation, the rim surfaces on the upper end of the outside section30, areas on the radially inward surface of the outside section 30 inthe section thereof above the center, the end surfaces on the upper sideof the teeth inner edge portions 311, and areas on the radially outwardsurface of the teeth inner edge portions 311 in the section thereofabove the center.

The second partition 324 is for the most part of the same form as thefirst partition 323, except that its shape is the reverse of the firstpartition 323, and it is provided with the second guide grooves 322.More specifically, it covers the lower section of the teeth 31, theunderside rim surfaces of the outside section 30 and the teeth inneredge portion 311, the radially inward surface of the outside section 30,and areas on the radially outward surface of the teeth inner edgeportion 311 in the lower section thereof. The insulators 32 formed bythe first partitions 323 and the second partitions 324 are sandwichedbetween the teeth inner edge portions 311 and the outside section 30.

FIGS. 8 and 9 are partially sectional views illustrating the firstpartition 323 and the second partition 324. The area shaded by parallelslanted lines indicates a sectional cut lying in the imaginary plane 300of FIGS. 2 and 3 (cf. FIG. 2). As shown in FIGS. 8 and 9, provided onthe (−Z) end of the first partition 323 (that is, the end toward thesecond partition 324) and on the (+Z) end of the second partition 324(that is, the end toward the first partition 323), are, respectively,taper surfaces 3231 a and 3241 a, provided to the outer sides of eachflank on the plurality of teeth 31, roughly parallel to the YZ plane,and taper surfaces 3231 b and 3241 b, roughly parallel to the ZX plane.When the first partition 323 and the second partition 324 are attachedto a tooth 31, the taper surfaces 3231 a and 3241 a, and the tapersurfaces 3231 b and 3241 b overlapping each other makes it possible toprevent a gap from arising between the first partition 323 and thesecond partition 324.

Of the two flanks on a tooth 31, along the outer side of the flank onwhich the conductor is wound heading from the second partition 324 endof the tooth toward the first partition 323 end, the first partition 323taper surface 3231 a is provided on the inner side surface of the firstpartition 323 (that is, the side facing the flank of the tooth 31), andthe second partition 324 taper surface 3241 a is provided on the outerside surface of the second partition 324 (that is, the side oppositethat which faces the flank of the tooth 31). Likewise, along the outerside of the tooth 31 flank on which the conductor is wound heading fromthe first partition 323 end of the tooth toward the second partition 324end, the first partition 323 taper surface 3231 a is provided on theouter side surface of the first partition 323, and the second partition324 taper surface 3241 a is provided on the inner side surface of thesecond partition 324.

The inclination direction and angle of the taper surfaces 3231 a and3231 b as seen from the first guide groove 321 side are made to be thesame as the respective inclination direction and angle of the tapersurfaces 3241 a and 3241 b as seen from the second guide groove 322side. This contributes to designing for shared use of parts (forexample, making in common part of the molding forms and part of thedesign data) required for the manufacture of the first partition 323 andsecond partition 324.

As shown in FIGS. 4 through 7, two protruding walls 3213 and 3214 areprovided on the upper end of the first partition 323 for preventing theconductor being layered in the (+Z) direction onto the plurality offirst guide grooves 321 from slipping out of form. The protruding wall3213 provided on the first guide groove 321 tooth inner edge portion 311side (that is, the teeth 31 center axis J1 side shown in FIG. 2) will bereferred to as the “first inner protruding wall 3213,” and theprotruding wall 3214 provided on the outside section 30 side (that is,on the opposite side to the center axis J1, sandwiching the teeth 31 andthe multiple first guide grooves 321) will be referred to as the “firstouter protruding wall 3214.” The first inner protruding wall 3213 andfirst outer protruding wall 3214 extend in the (+Z) direction of theconductor layering and have a convex shape facing the teeth 31 and theplurality of first guide grooves 321. A second inner protruding wall3223 and a second outer protruding wall 3224 are provided at the lowerend of the second partition 324 in order to prevent sideways slippage ofthe conductor layered in the (−Z) direction on the plurality of secondguide grooves 322.

FIGS. 10 through 12 illustrate a conductor having been wound onto aninsulator 32 to form a coil 35. As with FIGS. 4 through 7, these figuresfocus on a single insulator. FIGS. 10 and 11 are respectively a planview and a bottom view showing the condition in which a first layer 351has been formed on the coil 35. FIG. 12 is a plan view illustrating asituation in which a second layer 352 has been formed on the coil 35.

The circled reference numerals “1 s” through “8 s,” and “0” and “1 e”through “7 e” in FIG. 10 show the positions of the conductor at thestart point 3211 and end point 3212 in each of the first guide grooves321. The circled reference numeral “8 e” shows the position to which theconductor wound on the insulator 32 from the position marked as “8 s”(referred to below simply as “position 8 s”) is led along the (+X) sideof the insulator 32. Similarly, the circled reference numerals “1 s”through “8 s” and “1 e” through “8 e” show the positions of theconductor at each of the second guide groove 322 start points 3221 andend points 3222 shown in FIG. 5. The reference numerals cited above inFIGS. 10 and 11, as well as reference numerals “1 s” through “7 s” and“8,” and “0” and “1 e” through “7 e” in FIG. 12, indicate the conductorwinding sequence.

When a coil 35 is formed around the periphery of an insulator 32, theconductor is first led from position 0 in FIG. 10 toward position 1 s inFIG. 11, along the insulator 32 (+X) side face in the (−Z) directionparallel to the center axis J1, and is wound onto the insulator 32bottom end portion from position 1 s toward position 1 e in FIG. 11,along the second guide groove 322 (cf. FIG. 5). Next the conductor isled along the insulator 32 (−X) side face in the (+Z) direction parallelto the center axis J1, after which, by winding it from the 1 s positionto the 1 e position in FIG. 10 along the first guide groove 321 (cf.FIG. 4) and onto the upper end portion of the insulator 32, theconductor completes one revolution around the insulator 32. At the upperend portion of the insulator 32, the conductor is led by the first guidegroove 321 in a direction inclined with respect to the imaginary plane300 (cf. FIG. 2).

Next, the conductor passes sequentially from the FIG. 10 position 1 ethrough the FIG. 11 positions 2 s, 2 e, and FIG. 10 position 2 s, and iswound onto the FIG. 10 position 2 e. Thereafter, by similarly windingthe conductor onto the insulator 32 (and the teeth 31), a coil 35 firstlayer 351 is formed. The coil 35 first layer 351 conductor is wound 8times around the teeth 31 (hereafter a single turn of the conductor willbe referred to as “one row”), and is wound so that adjacent conductorsin the 8 rows contact one another with virtually no gap.

The coil 35 first layer 351 between positions 1 s and 1 e in FIG. 10contacts the first outer protruding wall 3214 (−X) side part 3216(referred to below as the “first layer contact part”), and betweenpositions 8 s and 8 e contacts the first inner protruding wall 3213 (+X)side part 3215 (referred to below as the “first layer contact part”).The first layer contact parts 3215 and 3216 are disposed along the coil35 first layer conductor winding direction (that is, approximatelyparallel to the conductor winding direction) at the upper end portion ofthe insulator 32; the first layer conductor is guided so as to followthe first contact part 3216 between positions 1 s and 1 e, and is guidedalong the first contact part 3216 between positions 8 s and 8 e. Thecoil 35 first layer 351, as shown in FIG. 11, also contacts the secondouter protruding wall 3224, and is guided parallel to the second outerprotruding wall 3224.

When formation of the coil 35 first layer 351 is complete, the conductorthat forms the coil 35 second layer 352 is guided sequentially along theinsulator 32 (+X) side, (−Z) side, and (−X) side to position 1 s.Thereafter, the conductor that forms the second layer 352 is wound in adirection crossing the first layer 351 conductor at the upper endportion of the insulator 32, so that the first layer 351 conductor andthe second layer 352 conductor cross at a crossing position 350. Inother words, at the upper end portion of the insulator 32, the firstlayer 351 conductor and the second layer 352 conductor have differinginclinations. In the present embodiment, the first layer 351 conductorhas a “positive inclination,” whereas the second layer 352 has a“negative inclination.”

As shown in FIG. 12, the winding-on position of the conductor in theinnermost row of the coil 35 second layer 352 (that is, the row on theinnermost side in the radial direction), is restricted by theinner-edge-portion guide groove 325 formed on the inner edge portion ofthe insulator 32 at positions 0 and 1 s in FIG. 12.

The second layer 352 conductor, by being guided to lay in contact on twofirst layer 351 conductors adjoining each other on the (+X) flank, thelower end portion, and the (−X) flank of the insulator 32, is wound ontothe insulator 32 in parallel with the first layer 351, and positioned atposition 2 s. That is, along the insulator 32 (+X) side and (−X) flanks,the second layer 352 conductor is accommodated in the hollow formed bytwo adjoining conductors from among the first layer 351 conductors whichare densely wound with virtually no gaps. Thus by being guided in thishollow, the conductor is wound onto the first layer 351 in such a waythat it is arrayed parallel to the first layer 351 and parallel to thecenter axis J1. Similarly, at the lower end portion of the insulator 32,the second layer 352 conductor is wound onto the first layer 351 suchthat it is arrayed parallel to the first layer 351 conductor andparallel to the imaginary plane 300 (cf. FIG. 3). Thereafter, theconductor is guided in a direction which crosses the first layer 351conductor so that it crosses the first layer 351 conductor at a crossingposition 350 and heads toward position 2 e; after passing throughposition 2 e, it is then wound in parallel to the first layer 351conductor and reaches position 3 s.

Thereafter, by winding the conductor in the same way onto the coil 35first layer 351, the second layer 352 is formed by the eight rows ofconductors arrayed in parallel to the first layer 351 conductor on eachface of the teeth 31 (a section (for example, the conductor on theradially innermost row) of the eight conductor rows may cross the 351conductor). In the coil 35 second layer 352 as well, the eight conductorrows are densely wound with virtually no gaps so that adjacentconductors contact one another.

Between the FIG. 12 positions 1 s and 1 e, the coil 35 second layer 352contacts the first inner protruding wall 3213 (−X) side part 3217 (the“second layer contact part” below), and between parts 7 s and 7 econtacts the first outer protruding wall 3214 (+X) side part 3218 (the“second layer contact part” below”). The second layer contact parts 3217and 3218 are disposed along the winding direction of the coil 35 secondlayer conductor in the upper end portion of the insulator 32 (that is,approximately parallel to the winding direction). A second layerconductor is guided between positions 1 s and 1 e along a second layercontact part 3217, and is guided between positions 7 s and 7 e along asecond layer contact part 3218. The coil 35 second layer 352 alsocontacts the second inner protruding wall 3223 (cf. FIG. 11), and isguided parallel to the second inner protruding wall 3223. As is clearfrom FIGS. 10 and 12, the sectional shape of the first outer protrudingwall 3214 takes a convex form toward the first guide groove 321 due tothe first layer contact part 3216 and the second layer contact part3218. The sectional shape of the first inner protruding wall 3213 takesa convex form toward the first guide groove 321 due to the first contactpart 3215 and the second layer contact part 3217.

With respect to the insulator 32, the angle and surface area formed bythe first inner protruding wall 3213 first contact part 3215 (cf. FIG.10) and the second layer contact part 3217 (cf. FIG. 12), and the angleand surface area formed by the first outer protruding wall 3214 firstlayer contact part 3216 (cf. FIG. 10) and the second layer contact part3218 (cf. FIG. 12) are given an appropriate size to fit the coil 35first guide groove 321 inclination or the number of coil 35 layers, etc.

Thereafter, the conductor is similarly wound until the coil reaches aspecified number of layers, and a stator 3 is formed by disposing thecoils 35 around each of the teeth 31 and the coils 35. FIG. 13 shows asection of the coil 35 on the insulator 32 side. As shown in FIG. 13, inthe present embodiment the coils 35 are formed on each of the insulators32 using a winding machine 9 (indicated by the phantom lines in FIG.13). The conductor is wound around the insulator 32 across four layerswithin a range such that it does not interfere with the winding machine9. The spacing between adjacent teeth 31 inner edge portions 311 isdetermined as the size at which the winding machine 9 nozzle can beinserted. The distance W between teeth inner edge portions 311 ispreferably greater than 3 mm and less than 4 mm (more preferably,greater than 3.2 mm and less than 3.5 mm; in the present embodiment itis 3.3 mm).

As explained above, plurality of first guide grooves 321 and pluralityof second guide grooves 322 to guide the coils 35 are formed in theupper and lower end portions of the insulators 32; the first guidegroove 321 is disposed at an inclination of just one pitch with respectto the imaginary plane 300 (cf. FIG. 2), and the second guide groove 322is disposed parallel to the imaginary plane 300. The end point 3212 of asingle first guide groove 321, the start point 3211 of another singlefirst guide groove 321 adjacent thereto on the (−Y) side, and thecorresponding second guide groove 322 start point 3221 and end point3222 are all positioned on a straight line parallel to the center axisJ1 (cf. FIG. 2).

The coil 35 second layer 352 conductor thus crosses the first layer 351conductor at a crossing position 350 along the upper end of the tooth31. Furthermore, by laying in contact on two conductors that, among thefirst layer 351 conductors densely wound with virtually no gaps onto theflanks of the teeth 31, adjoin each other, the second layer 352conductors are arrayed parallel to the first layer 351 conductor.Similarly, the third and fourth layer conductors cross the layers theycontact below along the upper end of the teeth 31 (that is, the secondand third layers), and are arrayed in parallel to the second and thirdlayer conductors on the flanks of the teeth 31.

As a result, crossing of conductors (for example, crossing of the firstlayer 351 conductor and the second layer 352 conductor) on both faces ofthe teeth 31 (that is, both faces of the insulator 32) during winding ofthe conductor from the second layer 352 forward can be prevented, andthe thickness of the coil 35 on both faces of the teeth 31 (the heightin the X direction from the insulator 32 face in FIG. 12) can bereduced. The coil 35 can thus be formed with a high slot-fill factor andthe stator 3 can be made compact. When the coil 35 is formed using awinding machine, contact can be prevented between the coil 35 alreadywound on an insulator 32 and the winding machine which is winding aconductor onto the adjacent 32, so that the coil 35 can be smoothlyformed.

The coil 35 second layer 352 conductor is also arrayed parallel to thefirst layer 351 conductor at the bottom end of the teeth 31 (as is truefor the third and fourth layers). Crossing of conductors at the bottomend of the teeth 31 can thus be prevented during winding of conductorsfrom the second layer 352 forward, and the coil 35 thickness in the Zdirection at the bottom end of the teeth 31 can also be reduced. As aresult, a further size reduction of the stator 3 is achieved, leading toa size reduction of the motor 1.

On the stator 3, guiding of the coil 35 by the first guide groove 321and the second guide groove 322 results in its disposition at aninclination with respect to the imaginary plane 300 at the top end ofthe teeth 31, making it parallel to the center axis J1 at each face ofthe teeth 31. It is thus possible to achieve tighter winding of the coil35 conductor with respect to the teeth 31 compared to the case when theconductor is inclined with respect to the center axis J1 on both facesof the teeth 31.

The coil 35 first layer 351 conductor is moved to the (−Y) side by onepitch width of the first guide groove 321 and second guide groove 322(that is, a distance approximately equal to the conductor diameter) witheach revolution around the perimeter of the insulator 32. As a result,the coil 35 first layer 351 can be constructed smoothly and at a highslot-fill factor with virtually no provision of spacing between adjacentconductors. By forming the second layer 352 and forward by furtherwinding of conductors on the first layer 351 which has been thus alignedat a high slot-fill factor, the coil 35 can be smoothly constructed andconductor mis-winding avoided. A high coil 35 slot-fill factor can thusbe achieved without adding complexity to the stator 3 manufacturingprocess.

On the stator 3 insulator 32, the first layer 351 winding is restrictedby the first guide groove 321 and the second guide groove 322, thereforewinding errors caused by the second layer 352 conductor becoming lodgedbetween first layer 351 conductors, etc. can be reliably prevented, andthe coil 35 can be smoothly constructed. That is, aligned winding can beachieved. Also, winding of the conductor row on the radially innermostside among the second layer 352 is restricted by the inner edge portionguide groove 325. At the radially inner end section of the teeth 31,therefore, second layer 352 conductor mis-winding caused by mis-windingof the radially innermost conductor row of the second layer 352 towardthe radial inside direction can be prevented, and the coil 35 can besmoothly constructed.

At the insulator 32, the winding position at the center axis J1 side andthe outside section 30 side of the plurality of layers of conductorswhich cross at the first guide groove 321 is guided by the first innerprotruding wall 3213 and first outer protruding wall 3214, which aremutually convex facing the plurality of first guide grooves 321.Therefore the coil 35 can be smoothly constructed even at the endsection of the plurality of first guide grooves 321 center axis J1 sideand the outside section 30 side. The first inner protruding wall 3213and the first outer protruding wall 3214 restrict the position anddirection of winding of the first layer 351 and second layer 352conductors by use of the first layer contact parts 3215 and 3216 and thesecond layer contact parts 3217 and 3218, thus achieving an alignedwinding of the first layer 351 and the second layer 352, so that alignedwinding of the coil 35 can be more easily realized.

On the insulator 32, the taper surfaces 3231 a and 3231 b are disposedon the second partition 324 side of the first partition 323, the tapersurfaces 3241 a and 3241 b are disposed on the first partition 323 sideof the second partition 324, and the taper surfaces 3231 a, 3231 boverlap with the taper surfaces 3241 a, 3241 b when winding onto thefirst partition 323 and second partition 324 teeth 31, thus enabling theprevention of gap formation between the first partition 323 and thesecond partition 324, while simplifying the assembly of the firstpartition 323 and the second partition 324.

Furthermore, on the insulator 32, of the two flanks on a tooth 31, alongthe flank on which the conductor is wound from the second partition 324section to the first partition 323 section, the taper surfaces 3231 aand 3241 a are provided on the inner side surface of the first partition323 and the outer side surface of the second partition 324. In turn,along the flank on which the conductor is wound heading from the firstpartition 323 section to the second partition 324 section the tapersurfaces 3231 a and 3241 a are provided on the outer side surface of thefirst partition 323 and the inner side surface of the second partition324. In other words, for the first partition 323 and the secondpartition 324, at the first guide groove 321 and second guide groove 322the taper surfaces 3231 a, 3231 b are formed on the outside of the faceon the start point 3211 and 3221-flanks, and the taper surfaces 3231 a,3231 b are formed on the inside of the face on the end point 3212 and3222-flanks. Therefore on the face at the side on which the conductor iswound from the top toward the bottom (that is, the face on the side ofthe first guide groove 321 end point 3212), the first partition 323 ispositioned on the outside of the second partition 324 in the area of thejoint between the first partition 323 and the second partition 324. Onthe face at the side on which the conductor is wound from the bottomtoward the top (that is, the face on the side of the second guide groove322 end point 3222), the second partition 324 is positioned on theoutside of the first partition 323 in the area of the joint between thefirst partition 323 and the second partition 324. As a result, catchingof the conductor (and the winding machine 9 nozzle) on the joint betweenthe first partition 323 and the second partition 324 during winding ofthe conductor on the teeth 31 can be prevented, and the coil 35 can besmoothly constructed.

Second Embodiment

FIGS. 14 and 15 are respectively a plan view and a bottom view showingthe region of an insulator 32 in a motor stator according to a secondembodiment of the present invention. In a motor according to the secondembodiment, a plurality of insulators 32 a is disposed in place of theplurality of insulators 32 shown in FIG. 1. Other structures are thesame as in FIG. 1, and the same reference numerals are used in theexplanation below.

In a motor according to a second embodiment, as with the motor 1according to the first embodiment, insulators 32 a are provided with afirst partition 323 and a second partition 324 respectively attached onthe (+Z) side and the (−Z) side of the teeth 31 (cf. FIG. 6). Aplurality of first guide grooves 321 is formed at the upper end portionof the first partition 323, and a plurality of second guide grooves 322is formed at the lower end portion of the second partition 324.

As shown in FIGS. 14 and 15, the difference in radial distance (that is,the inclination of the first guide groove 321) from the center axis J1(cf. FIG. 2) of the plurality of first guide groove 321 respective startpoints 3211 and end points 3212, and the difference in radial distancefrom the center axis J1 of the plurality of second guide groove 322respective start points 3221 and end points 3222 (that is, theinclination of the second guide groove 322) is equal to one half thepitch of the first guide groove 321 and the second guide groove 322. Theouter protruding wall and inner protruding wall corresponding to thefirst partition 323 outside section 30 and teeth inner edge portion 311extend in the (+Z) direction and have a sectional shape which is convextoward the first guide groove 321, as in the first embodiment. Theinsulator 32 a is formed so as also to be convex toward the second guidegroove 322 for the second partition 324 outer protruding wall and innerprotruding wall.

Also, similar to the insulator 32 shown in FIGS. 4 and 5, for theplurality of first guide grooves 321 and second guide grooves 322, therespective single first guide groove 321 end point 3212 and singlesecond guide groove 322 start point 3221 are positioned on a straightline extending in the Z direction (that is, on a straight line parallelto the center axis J1). The guide groove 322 end point 3222 and thestart point 3211 of another first guide groove 321 adjacent to the firstguide groove 321 on the (−Y) side are positioned on a straight lineextending in the Z direction. As with the motor 1 according to the firstembodiment, in the motor according to a second embodiment a coil 35 isconstructed by winding plurality of conductor layers on each insulator32 a to form a stator 3.

In the motor stator according to the second embodiment, the shape of theinsulator 32 a in the region of the first guide groove 321 and thesecond guide groove 322 can be made the same by making the inclinationof the first guide groove 321 and the second guide groove 322 equal. Asa result, the same parts can be used to manufacture the insulator 32 afirst partition 323 and the second partition 324.

In the stator according to the second embodiment, because the coil 35conductor is guided by the first guide groove 321 and the second guidegroove 322, the conductor during winding of the second layer 352conductor crosses the first layer 351 conductor at the upper and lowerends of the teeth 31 and is wound onto the teeth 31. Along the flanks ofthe teeth 31, the first layer 351 conductor and the second layer 352conductor are aligned so as to be parallel to one another. By thismeans, as with the first embodiment, crossing of conductors on the twofaces of the teeth 31 is prevented when winding the second layer 352 andsubsequent conductor layers, and the thickness of the coil 35 on bothfaces of the teeth 31 can be reduced. By making the coil 35 conductorparallel to the center axis J1 on each face of the teeth 31, theconductor can be wound more densely with respect to the teeth 31.

Again, as with the first embodiment, by moving the coil 35 first layer351 conductor by one pitch only to the (−Y) side for each revolutionaround the periphery of the insulator 32 a, there are virtually no gapscreated between adjoining conductors, and the coil 35 can be smoothlyconstructed with a high slot-fill factor. Because the conductor windingis restricted by the first guide groove 321 and the second guide groove322 in the insulator 32 a, conductor mis-winding can be reliablyprevented and the coil 35 smoothly constructed.

Third Embodiment

FIGS. 16 and 17 are partially sectional views showing one insulator'sfirst partition 323 a and second partition 324 a in a motor statoraccording to a third embodiment of the present invention. The “shadedarea indicated by parallel slanted lines, as in FIGS. 8 and 9, is asection through an imaginary plane 300 (cf. FIG. 2) which is parallel tothe center axis J1 and perpendicular to the teeth 31. Each of theinsulators in the motor according to the third embodiment is the same asthose in FIG. 1 with the exception that the sectional shape of the endsection differs slightly in the direction parallel to the center axisJ1. The same reference numerals are used in the explanation below.

As shown in FIG. 16, in the first partition 323 a, a first crown section3232, which is the most (+Z)-ward, distal edge of the end portion on thepartition side where the first guide grooves 321 are formed, ispositioned closer to the end point side than the start point side of theconductor winding (that is, closer to the first guide groove 321 endpoint 3212 than the start point 3211). As shown in the FIG. 18 expandedview, the cross-sectional form of the end portion on the (+Z) side ofthe first partition 323 a has a larger radius of curvature R1 on the(+X) side (that is, on the first guide groove 321 end point 3212 side)than the radius of curvature R1 on the (−X) side (that is, on the firstguide groove 321 start point 3211 side). In the present embodiment, theradius of curvature R1 is set at approximately 3 mm, and the radius ofcurvature R2 is approximately 4.1 mm.

As shown in FIG. 17, in the second partition 324 a as well, a secondcrown section 3242, which is the most (−Z)-ward, distal edge of the endportion on the partition side where the second guide grooves 322 areformed, is positioned closer to the start point side than the end pointside of the conductor winding (that is, closer to the start point 3221than the end point 3222 of the plurality of second guide grooves 322).In the cross-sectional form of the second partition 324 a, the radius ofcurvature on the end point 3222 side is greater than that on the startpoint 3221 side.

As explained above, in the motor insulator 32 according to the thirdembodiment, the bending of parts from the crown section of the firstguide groove 321 and the second guide groove 322 up to the winding endpoint is kept gradual, without changing the width of the first guidegroove 321 and the second guide groove 322 in the X direction. Thismakes it possible to stop the conductor from separating from theinsulator 32 in the region around the first guide groove 321 end point3212 and in the region of the second guide groove 322 end point 3222and, by preventing mis-winding of the conductor, to smoothly constructthe coil 35. The thickness of the coil 35 on both faces of the insulator32 can also be reduced. This structure which makes it possible to keepseparation of the conductor from the insulator 32 under control isparticularly suited to a stator in which the axial length of the teeth31 is longer than the width thereof, and in which the radius ofcurvature at the top and lower end portions of the insulator 32 isrelatively small.

It will be appreciated that it is acceptable for the end section atwhich the radius of curvature varies to be only one of the end sectionsfrom the first guide groove 321 side and the second guide groove 322side. “Radius of curvature” at the start point and the end point sidesrefers to an approximate radius of curvature. For example, it can beinterpreted as an average radius of curvature, and can also beinterpreted as a minimum radius of curvature on the start point and endpoint sides.

While several preferred embodiments of the present invention have beenexplained, the present invention is not limited to the foregoingembodiments, and may be variously modified.

For example, for the stator 3 insulator, the first guide groove 321 canbe set to be parallel to the imaginary plane 300, and the second guidegroove 322 can be disposed so as to be inclined by just one pitch withrespect to the imaginary plane 300, so that the coil 35 is formed suchthat conductors cross only at the bottom end of the teeth 31.

In the stator 3, a high slot-fill factor coil 35 can be constructed bywinding the coil 35 conductor in such a way that it is parallel to thecenter axis J1 on each face of the teeth 31 and moves by just one pitchunit for each revolution around the insulator. Thus from the standpointof building the coil 35 at a high slot-fill factor, the total respectiveinclinations of, for example, the first guide groove 321 and the secondguide groove 322 do not have to be equal to the pitch. For example, therespective inclinations of the first guide groove 321 and the secondguide groove 322 could be set at a 0.7× and 0.3× the pitch.

From the standpoint of improving the coil 35 slot-fill factor, it ispreferable that the inclination of the first guide groove 321 and thesecond guide groove 322 be greater than 0, and that it be less than thepitch of the first guide groove 321 and the second guide groove 322. Inother words, it is preferable to make the difference in distance fromthe center axis J1 to the first guide groove 321 start point 3211 andend point 3212, and the difference in distance from the center axis J1to the second guide groove 322 start point 3221 and end point 3222 beless than the pitch of the first guide groove 321 and the second guidegroove 322.

For the stator 3, speaking only from the standpoint of reducing the coil35 thickness at both faces of the teeth 31, it is sufficient for thevarious coil 35 layers to be mutually aligned in parallel and to bewound along approximately the center axis J1; they may be slightlyinclined with respect to the center axis J1. Therefore when the coil 35is formed so that the conductors cross only at the top end (and/or thebottom end) of the teeth 31, then for example the first guide groove 321and the second guide groove 322 may be disposed in parallel to theimaginary plane 300. In that case, the coil 35 conductor is disposed soas to be slightly inclined with respect to the center axis J1 at bothsides or at one side of the teeth 31. For a conductor inclination atboth faces of the teeth 31, that inclination is equal to one pitch unitof the first guide groove 321 and second guide groove 322.

A reduction in the coil 35 thickness can be achieved on both faces ofthe teeth 31 even in cases where, for example, one or two rows of aconductor among the eight conductor rows of the coil 35 second layer 352cross the first layer 351 conductor on the various faces of the teeth31. Thus, by virtue of the majority of conductor rows in each layerbeing mutually aligned in parallel with the conductors in adjacentlayers on each face of the teeth 31, the thickness of the coil 35 can bereduced on both faces of the teeth 31, even in cases in which someconductors cross on the face of teeth 31 with the conductors of anadjacent layer.

A guide groove which joins the first guide groove 321 start point 3211and the second guide groove 322 end point 3222, and a guide groove whichjoins the first guide groove 321 end point 3212 and the second guidegroove 322 start point 3221 may be constructed on a face of theinsulator. The conductor which forms the coil 35 first layer 351 mayalso be wound from the center axis J1 side to the outside section 30side with respect to the insulator.

In cases where it is possible to prevent mis-winding of the conductorand smoothly construct the coil 35 by slackening the conductor windingto some degree with respect to the teeth 31 so as to cause theconductors to cross only at the top end (and/or bottom end) of the teeth31, the insulator first guide groove 321 and/or second guide groove 322may be omitted. In cases where mis-winding of the conductor row on theradially innermost side of the second layer 352 is reliably prevented bycontact, etc. with the first inner protruding wall 3213, the insulator32 inner edge portion guide groove 325 may be omitted.

Insulators attached to plurality of (nine in the present preferredembodiment) teeth 31 may be comprised of nine first partitions 323connected or integrally formed in an annular shape, and nine secondpartitions 324 connected or integrally formed in an annular shape. Theinsulator is not limited to being formed by attachment of a resin-moldedfirst partition 323 and second partition 324 to the teeth 31 and could,for example, be formed by molding a thermoplastic resin for the teeth31. In such cases, the plurality of teeth 31 could be said to be coveredby a single insulator. With such an insulator, plurality of first guidegrooves 321 are formed on the upper end portions which cover therespective top ends of the plurality of teeth 31 (that is, therespective upper end portions of the plurality of parts which cover therespective plurality of teeth 31), and plurality of second guide grooves322 are formed at the bottom ends which cover the respective bottom endsof the plurality of teeth 31.

The stator 3 may be a segmented type in which laminar componentsincluding the individual teeth 31 are appropriately situated—forexample, in which in between adjoining teeth 31 the outside section 30is discontinuous.

Motors provided with the stator 3 are not limited to inner rotor types;they may also be of the outer rotor type.

1. An electric motor stator comprising: a plurality of teeth disposed ina radial form centered about a predetermined center axis; insulatorscovering said plurality of teeth; and a plurality of coils provided oneach of said plurality of teeth by winding conductors onto saidinsulators through to multiple layers; wherein at one axial end of theplurality of areas where said insulators cover each of said plurality ofteeth, the radius of curvature of the conductor windings on the endingside is larger than the radius of curvature on the starting side.
 2. Amotor stator as set forth in claim 1, wherein the axial length of eachof said plurality of teeth is longer than the widthwise span orthogonalto the teeth radially as centered on the center axis.
 3. A motor statoras set forth in claim 1, wherein a plurality of edge grooves forgoverning the wind-on position of the conductors in the first layer ofsaid coil is provided in insulator edge portions where upper endsurfaces, being upper ends of each of said insulators, and flanks ofsaid insulators, covering each flank of said plurality of teeth, areconnected to each other.
 4. A motor stator as set forth in claim 1,wherein along an upper end portion of said insulators that covers anaxially upper end of each of said plurality of teeth, said insulatorsare provided with a plurality of upper guide grooves rowed parallel toeach other at a pitch approximately equal to the diameter of theconductors, for governing the wind-on position of the conductors.
 5. Amotor stator as set forth in claim 4, wherein: said insulators, in lowerend portions thereof covering the lower ends of each of said pluralityof teeth, are further provided with a plurality of guide grooves, rowedin parallel at said pitch, for governing the wind-on position of theconductors; and in the conductors as wound, the end of a one of theupper guide grooves and the beginning of a one of the lower guidegrooves are positioned on a straight line parallel to the center axis,and the end of a one of the lower guide grooves and the beginning ofanother of the upper guide grooves adjoining said one of the upper guidegrooves are positioned on a straight line parallel to the center axis.6. An electric motor comprising: a first assembly having a stator as setforth in claim 1; a second assembly disposed opposing said stator, andhaving a field magnet for generating between said second assembly andsaid stator rotational force centered on the center axis; and a bearingmechanism for supporting said second assembly to enable it to rotaterelative to said first assembly, centered on the center axis.
 7. Anelectric motor stator comprising: a plurality of teeth disposed in aradial form centered about a predetermined center axis; insulatorsrespectively covering said plurality of teeth, said insulators beingcomposed of first partitions for covering a first half of said pluralityof teeth from the center axially along the teeth, and second partitionsfor covering a second half of said plurality of teeth from the centeraxially along the teeth, wherein said first partitions, along the outerside of each of flanks, parallel to the center axis, of said pluralityof teeth, being provided with a first taper surface located on asecond-partition-directed end thereof; and said second partitions, alongthe outer side of each flank of said plurality of teeth, being providedwith a second taper surface located on a first-partition-directed endthereof, and making an overlapping fit with said first taper surface;and a plurality of coils provided on each of said plurality of teeth bywinding conductors onto said insulators through to multiple layers.
 8. Amotor stator as set forth in claim 7, wherein: along the one flank ofeach tooth on which said conductor is wound to head from said first halftoward said second half, said first taper surface is disposed on theinner-side surface of said first partition opposing that tooth flank,and said second taper surface is disposed on the outer-side surface ofsaid second partition, opposite the side thereof opposing that toothflank; and along the other flank of each tooth on which said conductoris wound to head from said second half toward said first half, saidfirst taper surface is disposed on the outer-side surface of said firstpartition, and said second taper surface is disposed on the inner-sidesurface of said second partition.
 9. A motor stator as set forth inclaim 7, wherein the axial length of each of said plurality of teeth islonger than the widthwise span orthogonal to the teeth radially ascentered on the center axis.
 10. An electric motor comprising: a firstassembly having a stator as set forth in claim 7; a second assemblydisposed opposing said stator, and having a field magnet for generatingbetween said second assembly and said stator rotational force centeredon the center axis; and a bearing mechanism for supporting said secondassembly to enable it to rotate relative to said first assembly,centered on the center axis.